Snow forecasts are generally made in terms of inches of
accumlated snow. However, precipitation is measured by
the amount of liquid water. How do you measure liquid
water when it's all snow?

Answer: you melt it. Just take what falls in the rain
gauge, melt it, and see how much water there is. Simple
procedure, but it does require human intervention.
Unfortunately, as we move closer to a fully automated
observing network, we lose the ability to determine the
amount of water associated with snowfall.

The automated techniques don't work too well for measuring
snow in its raw form, either. The standard technique is
to use a level surface, stick a ruler in the snow and see
how deep it is, and then brush off the surface to allow
the next batch of snow to accumulate. Try that with a cheap,
automated device, and then have it try to distinguish
between snow that fell and snow that drifted.

The total depth of snow added to the ground is rarely as
large as the amount of snow that falls. Several things
happen. First, as snow first hits the ground, it might
melt. Eventually, if it snows hard enough, the snow
starts "sticking". Second, as snow gets added to the
snowpack it compresses the snow beneath it, so six inches
plus six inches might only equal ten inches. Finally,
if some rain is mixed in it might melt some of the snow,
which then might re-freeze into a hard crust.

All of these processes suggest that as snow lies on the
ground, it becomes denser and denser. But how much water
is in fresh snow? A good average number is that
about a foot of snow is equivalent to one inch of water.
But this number is highly variable, and in particular is
strongly dependent on temperature. If the temperature is
above freezing, an inch of liquid might correspond to
six inches of snow. This is called wet snow.
If the temperature is around 30 degrees, ten inches of snow
might equal an inch of rain. And if the temperature is
only 10 degrees Fahrenheit, it might take 18 to 24 inches
of snow to equal one inch of liquid water. This is called
dry snow, or powder snow.

Snow forecasts are made by first forecasting the amount
of equivalent liquid water that will fall. The numerical
weather prediction models give you precipitation in liquid
form, in hundredths of inches. Next, the forecaster takes
the forecasted precipitation amount, decides how much of it
will melt before sticking, converts the rest to an equivalent
snow depth based on the temperature at which the snow will
be accumulating, and comes out with the snow forecast.

Try it yourself. Use the precipitation forecasts and
temperature forecasts contained in this FOUS bulletin
from Bangor, Maine on 12Z Nov. 14, 1997 to predict the
total accumulated snowfall due to the upcoming storm: